Stellar populations -- the next ten years

The study of stellar populations is a discipline that is highly dependent on both imaging and spectroscopy. I discuss techniques in different regimes of resolving power: broadband imaging (R~4), intermediate band imaging (R~16, 64), narrowband spectral imaging (R~256, 1024, 4096). In recent years, we have seen major advances in broadband all-sky surveys that are set to continue across optical and IR bands, with the added benefit of the time domain, higher sensitivity, and improved photometric accuracy. Tunable filters and integral field spectrographs are poised to make further inroads into intermediate and narrowband imaging studies of stellar populations. Further advances will come from AO-assisted imaging and imaging spectroscopy, although photometric accuracy will be challenging. Integral field spectroscopy will continue to have a major impact on future stellar population studies, extending into the near infrared once the OH suppression problem is finally resolved. A sky rendered dark will allow a host of new ideas to be explored, and old ideas to be revisited.Comment: Invited review, IAUS 241, "Stellar Populations as Building Blocks of Galaxies," eds. Vazdekis, Peletier. 12 pages, 1 table. (The sideways table should print ok; there are 10 columns.

Episodic starbursts in dwarf spheroidal galaxies: a simple model

Dwarf galaxies in the Local Group appear to be stripped of their gas within 270 kpc of the host galaxy. Color-magnitude diagrams of these dwarfs, however, show clear evidence of episodic star formation (\Delta{}t ~ a few Gyr) over cosmic time. We present a simple model to account for this behaviour. Residual gas within the weak gravity field of the dwarf experiences dramatic variations in the gas cooling time around the eccentric orbit. This variation is due to two main effects. The azimuthal compression along the orbit leads to an increase in the gas cooling rate of ~([1+\epsilon]/[1-\epsilon])^2. The Galaxy's ionizing field declines as 1/R^2 for R>R_disk although this reaches a floor at R~150 kpc due to the extragalactic UV field ionizing intensity. We predict that episodic SF is mostly characteristic of dwarfs on moderately eccentric orbits (\epsilon>0.2) that do not come too close to the centre (R>R_disk) and do not spend their entire orbit far away from the centre (R>200 kpc). Up to 40% of early infall dwarf spheroidals can be expected to have already had at least one burst since the initial epoch of star formation, and 10% of these dwarf spheriodals experiencing a second burst. Such a model can explain the timing of bursts in the Carina dwarf spheroidal and restrict the orbit of the Fornax dwarf spheroidal. However, this model fails to explain why some dwarfs, such as Ursa Minor, experience no burst post-infall.Comment: 8 pages, 8 figures. ApJ accepte

The Galaxy in Context: Structural, Kinematic and Integrated Properties

Our Galaxy, the Milky Way, is a benchmark for understanding disk galaxies. It is the only galaxy whose formation history can be studied using the full distribution of stars from white dwarfs to supergiants. The oldest components provide us with unique insight into how galaxies form and evolve over billions of years. The Galaxy is a luminous (L-star) barred spiral with a central box/peanut bulge, a dominant disk, and a diffuse stellar halo. Based on global properties, it falls in the sparsely populated "green valley" region of the galaxy colour-magnitude diagram. Here we review the key integrated, structural and kinematic parameters of the Galaxy, and point to uncertainties as well as directions for future progress. Galactic studies will continue to play a fundamental role far into the future because there are measurements that can only be made in the near field and much of contemporary astrophysics depends on such observations.Comment: 69 pages, 18 figures, LaTeX. See http://www.physics.usyd.edu.au/~jbh/S/ARAA_2016.pdf for published versio

Measuring Outer Disk Warps with Optical Spectroscopy

Warps in the outer gaseous disks of galaxies are a ubiquitous phenomenon, but it is unclear what generates them. One theory is that warps are generated internally through spontaneous bending instabilities. Other theories suggest that they result from the interaction of the outer disk with accreting extragalactic material. In this case, we expect to find cases where the circular velocity of the warp gas is poorly correlated with the rotational velocity of the galaxy disk at the same radius. Optical spectroscopy presents itself as an interesting alternative to 21-cm observations for testing this prediction, because (i) separating the kinematics of the warp from those of the disk requires a spatial resolution that is higher than what is achieved at 21 cm at low HI column density; (ii) optical spectroscopy also provides important information on star formation rates, gas excitation, and chemical abundances, which provide clues to the origin of the gas in warps. We present here preliminary results of a study of the kinematics of gas in the outer-disk warps of seven edge-on galaxies, using multi-hour VLT/FORS2 spectroscopy.Comment: 7 pages, 7 figures; to appear in the proceedings of IAU Symposium 254 "The Galaxy disk in a cosmological context", Copenhagen, June 200

Microslit Nod-shuffle Spectroscopy - a technique for achieving very high densities of spectra

We describe a new approach to obtaining very high surface densities of optical spectra in astronomical observations with extremely accurate subtraction of night sky emission. The observing technique requires that the telescope is nodded rapidly between targets and adjacent sky positions; object and sky spectra are recorded on adjacent regions of a low-noise CCD through charge shuffling. This permits the use of extremely high densities of small slit apertures (`microslits') since an extended slit is not required for sky interpolation. The overall multi-object advantage of this technique is as large as 2.9x that of conventional multi-slit observing for an instrument configuration which has an underfilled CCD detector and is always >1.5 for high target densities. The `nod-shuffle' technique has been practically implemented at the Anglo-Australian Telescope as the `LDSS++ project' and achieves sky-subtraction accuracies as good as 0.04%, with even better performance possible. This is a factor of ten better than is routinely achieved with long-slits. LDSS++ has been used in various observational modes, which we describe, and for a wide variety of astronomical projects. The nod-shuffle approach should be of great benefit to most spectroscopic (e.g. long-slit, fiber, integral field) methods and would allow much deeper spectroscopy on very large telescopes (10m or greater) than is currently possible. Finally we discuss the prospects of using nod-shuffle to pursue extremely long spectroscopic exposures (many days) and of mimicking nod-shuffle observations with infrared arrays.Comment: Accepted for publication in PASP; 25 pages, 12 figures. A higher-quality compressed Postscript file (2.2Mb) is available from http://www.pha.jhu.edu/~kgb/papers/nodshuffle2000hq.ps.g